1. Field of the Invention
This invention relates generally to image sensors. In particular, it relates to a design for image sensor microlenses to improve uniformity of effective incident light at different regions,
2. Description of the Related Art
Solid state image sensors are necessary components in many optoelectronic devices, including digital cameras, cellular phones, and toys. Conventional image sensors Include both charge coupled device (CCD) image sensors and complementary metal oxide semiconductor (CMOS) image sensors. The basic technology used to form the CMOS image sensor is common to both types sensors.
The CMOS image sensor comprises a photo detector for detecting light and a logic circuit for converting the detected light into an electric signal representing data regarding the detected light. The fill factor, sometimes referred to as the aperture efficiency, is the ratio of the size of the light-sensitive area to the size of the total pixel size. Although efforts have been made to increase the fill factor of the image sensor and thereby increase the sensor sensitivity, further increases in the fill factor are limited because the associated logic circuitry cannot be completely removed. Accordingly, in order to increase the sensitivity of the light, a microlens formation technology has been used to converge and focus the incident light onto the photo detector by changing the path of the light that reaches the lens of the photo detector. In order for the image sensor to detect and provide a color image, it typically must include both a photo detector for receiving the light and generating and accumulating charge carriers and a color filter array (CFA), i.e., a plurality of color filter units sequentially arranged above the photo detector The CFA typically uses one of two alternative three-color primary configurations, either red R, green G and blue B (PCB) or yellow Y, magenta X and cyan C (CMY). A plurality of microlenses are positioned above the color filter array to increase the photo-sensitivity of the image sensor.
In the followings a conventional CCD image device will be explained.
FIG. 1 is a cross-sectional view showing a conventional solid-state image device. In FIG. 1, reference numeral 13 represents a semiconductor substrate provided with a solid-state image sensor; 12 represents a p-well formed in the semiconductor substrate 13; 11 represents a photodiode; 10 represents a charge transfer part; 9 represents a silicon oxide or nitride film; 8 represents a polysilicon electrode; 14 represents a photo-shielding metal layer; 25 represents a surface protective coating of semiconductor elements; 19 represents a planarization layer for setting elements thereon; 24 represents a color filter array; 23 represents an intermediate transparent film; and 21 represents microlenses. Furthermore, another conventional example comprises one additional layer of metal film formed via a silicon oxide film on the photo-shielding metal layer 14 for strengthening the photo-shielding and forming a semiconductor element with the surface protective coating 25. A microlens 21 is adjusted and positioned corresponding to each photodiode, and light converged by the lens is directed to the photodiode 11 to enhance sensitivity. Among electrons and holes arising from photoenergy in the photodiode 11, the electrons are forwarded to the charge transfer part 10 by voltage applied to the polysilicon electrode 8. The transferred electrons are then forwarded to an output part by potential energy created in the charge transfer part 10 through the voltage applied to the polysilicone electrode 8.
Examples of various forms of the solid state sensor structures are to be found in the prior art. Okamoto (U.S. Pat. No. 6,545,304 B2) discloses a photoelectric converter element group on one section of a semiconductor substrate and a charge transfer path to transfer accumulated signal charge to a contiguous readout gate region having a readout gate electrode associated therewith. Umetsu et al. (U.S. Pat. No. 6,528,831 B2) discloses a solid state image pickup device in which a matrix array of photoelectric sensors are formed adjacent to charge transfer channels and wherein a read-cum-transfer electrode is formed on an insulating layer and surrounds each photoelectric element. These devices are cited here as examples of a CCD type sensor device.
In general, the image sensor is built in a chip, and the microlenses corresponding to the photo detectors are arranged in a matrix. The solid state image sensor is placed where light is converged by an optical lens and an image is formed. However, the image captured on the edge region of the matrix is darker than that in the center region.
As shown in FIG. 2, when the incident light P0 transmits into the microlens 21 and through the stacked transmission layer comprising the color filter layer 27 and an IC transparent stacked layer 29 in the left pixel P0 with a chief angle θ=0°, the incident light R0 is focused on the sensing area 11 of the photo detector. This ideal situation of a chief angle θ=0° occurs at the center region of the sensor chip 10. But if the incident light R1 transmits into the microlens 21 with a chief angle θ other than 0°, the incident light reaching the photo detector may shift outside the sensing area 11. The phenomenon is especially problematic for microlenses 21 disposed near the edge region of the sensor chip 10. The pixel P1 shown in the middle of FIG. 2 is between the center region and the edge region of the sensor chip 10, and the right pixel P2 is arranged in the edge region. In pixels P1 and P2, the ideal incident light I is uniformly collimated light shown by dashed lines, while the real incident light R1 and R2 is shown by solid lines transmitted into the microlens 21 with chief angles θ1 and θ2, and θ1<θ2. Therefore, the sensing area 11 corresponding to the incident light R1 and R2 transmitted with a chief angle θ>0° obtains less photoenergy than the sensing area 11 corresponding to the incident light R0 transmitted with a chief angle 0°.
The traditional method to resolve the issue of shift of the focus center is shifting the microlens 21 to correct the focus center of the incident light within the sensing area 11. As shown in FIG. 3, the microlens 21′ before shifting is aimed at the sensing area 11R of the right pixel PR and the incident light Rb passing though the color filter layer 27R falls outside the sensing area 11R. After shifting, the shifted microlens 21 can focus the incident light Rf on the right pixel PR. However, microlenses 21 near the boundary of the chip may be shifted so much that the incident light Rf passes through the adjacent color filter layer 27L and cross-talk phenomenon CT occurs in the adjacent pixels PL and PR.